A load impedance, such as, for example, a plasma impedance, can be influenced by many parameters and therefore can vary over time. Thus, it can be difficult to match such a variable load impedance to an HF source. Automatic tuning circuits are sometimes used to improve matching of the load impedance in plasma thin-film technology for surface treatments. The automatic tuning circuits continuously follow changes in the load impedance in the plasma process.
Load impedances of laser excitation plasmas tend to fluctuate less than load impedances for plasma thin-film technologies Thus, fix match circuits can be used for load impedances of laser excitation plasmas. These fix match circuits are fixedly set for an acceptable load matching under standard burning (or ignited) conditions.
If the HF power from the source is supplied in a pulsed mode and the power is regulated by influencing the pulse duty factor, the load impedance fluctuation problem can be exacerbated since a periodic change between the impedance during the burning state and the impedance during ignition of the plasma also exists in the switched-on state.
Consequently, a compromise between optimization of the load matching and the ignition behavior is frequently set as matching. In addition, an ignition auxiliary circuit is usually used.